T NBS

Pubii - cations

A11101 7 c? fl 3 5 q

A111DM T 3 ( fl IMBSIR 79-1778

A Perspective of a Workshop

on Stability of (Thin Film) Solar

Cells and Materials

H. A. Schafft and D. E. Sawyer

Electron Devices Division

Center for Electronics and

Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

June 1 979

Issued July 1 979

Prepared for Advanced Materials R&D Branch

Division of Distributed Solar Technology

“ Ul ———j Department of Energy

jLqq Washington, DC 20545

. U56

73-1778

SEP 1 7 19'

NBSIR 79-1778

A PERSPECTIVE OF A WORKSHOP

ON STABILITY OF (THIN FILM) SOLAR

CELLS AND MATERIALS

H. A. Schafft and D. E. Sawyer

Electron Devices Division

Center for Electronics and

Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

June 1 979

Issued July 1 979

Prepared for Advanced Materials R&D Branch

Division of Distributed Solar Technology

Department of Energy

Washington, DC 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

A Perspective of a Workshop on Stability of

(Thin Film) Solar Cells and Materials

V Harry A. Schafft

and

David E. Sawyer

Abstract

The results of a workshop on the Stability of (Thin Film) Solar Cells

and Materials are reviewed from a measurements perspective. Solar

cells of the following three material groupings were considered at the

workshop: (1) CU2 S/ [ CdZn] S , Cu-ternaries/CdS , InP/CdS, and amorphous

Si; (2) polycrystalline, MIS, and conducting-oxide Si; and, (3)

polycrystalline and AMOS GaAs . Considering the relative state of

immaturity of these developing cells and the goal for high reliability

and stability, two general areas of work are recommended. One is to develop an improved understanding of cell operation and of component

structures of these cells. The other is to develop an improved

measurements base. Specific needs and recommendations are provided.

Key Words: Accelerated tests; failure mechanisms; failure modes;

photovolt aics ; reliability; semiconductors; solar cells; stability;

testing; thin films.

1. Introduction

A workshop on the Stability of (Thin Film) Solar Cells and Materials was conducted by

the National Bureau of Standards at its Gaithersburg, Maryland, site on May 1 to 3. 1978,

for the Department of Energy's Advanced Materials Research and Development Branch.

Three groups of exploratory materials and device concepts for solar cells were considered:

(1) CU2 S/ [ CdZn] S , Cu-ternaries/CdS, InP/CdS, and amorphous Si, (2) polycrystalline,

metal-insulator-semiconductor (MIS), and conducting-oxide Si, and (3) polycrystalline and

antireflection-coated metal-oxide-semiconductor (AMOS) GaAs.

The basic questions asked of the workshop participants were: (1) what are the

obstacles to achieving the necessary stability and reliability of thin-film solar cells and

(2) what needs to be done to overcome these obstacles? They responded by relating their

experiences with the many new and developing thin film cells, by reviewing the status of the

knowledge of cell operation and cell degradation, and by identifying the many gaps in the

applicable device, processing, and measurement technologies.

This report supplements the formal report of the workshop. 2 it considers the

results of the workshop from the perspective of measurement requirements related to the

achievement of cell stability and reliability and integrates the recommendations made and

needs identified by the participants of the workshop in the context of this perspective.

The highlights from the workshop report are reprinted here as an appendix for reference by

the reader.

2. Implications of High Reliability and Stability Goals

With their papers and discussions, the workshop participants revealed how convoluted is

the task of achieving the high reliability and stability goals for thin film solar cells.

Given the status of the technology, a coordinated effort on a number of fronts is required.

Consider why this is so.

^In support of the Department's Photovoltaic Program.

2 Sawyer, D. E., and Schafft, H. A., Semiconductor Measurement Technology : NBS/DOE

Workshop, Stability of (Thin Film) Solar Cells and Materials, NBS Special Publication

400-58 (August 1979).

1
To achieve these goals requires the use of accelerated tests to measure reliability and

stability and therefrom to measure improvements of these performance attributes as a result

of changes in materials, design and manufacture. The workshop participants accepted this

observation as a given. What are the implications of such a reliance on accelerated tests?

It was pointed out that there are two underlying assumptions to the meaningful use of

accelerated tests which are often overlooked and in the present case are not adequately

satisfied.

One assumption is that information exists about what stresses to impose in these tests. Knowing what stresses to impose comes from a knowledge of which degradation mechanisms

operate in these solar cells and under what conditions. To learn which of these mechanisms

are active requires both a thorough understanding of the principles of cell operation, an understanding of component materials, and the existence of a data base on cell use.

The other assumption is that the cells to be tested have been fabricated under

conditions where the materials and processes are under sufficient control that reproducible

cells are being produced. To be able to do this requires an understanding of the principles

of cell operation and an understanding of how variations in material characteristics and

fabrication processes can affect cell performance.

Implicit to the requirements needed to satisfy the above assumptions is the existence

of an adequate measurements base. This base provides the measurement tools to characterize

and learn about cell materials, fabrication, and operation; it provides the tools and

procedures to develop a history of cell experience that can be used to compare and evaluate

different cells and fabrication processes; it provides the measurement tools to study and

understand cell degradation from which an intelligent selection of stresses can be made for use in accelerated tests; and it provides the test procedure for accelerated tests to achieve the measurement precision needed to predict reliability and stability with

sufficient accuracy.

The workshop participants considered these assumptions which underlie meaningful accelerated

tests and found them wanting. They reported that many degradation mechanisms are not understood in sufficient detail to be able to design adequate accelerated tests; the

structure and principle of operation are not adequately understood for many cells; and the

effects of material characteristics and cell processing steps on cell performance are also

not sufficiently understood. In addition, standard measurement methods and procedures to characterize materials and cell components and minimum standards for reporting characterization and accelerated test data are all either inadequate or nonexistent.

3. An Integration of Needs

Two major areas of work emerge as ones that require serious attention and support.

This conclusion follows after an examination of the participants' statements about (1) the

requirements for realizing the goals for cell stability and realibility, (2) how well the

thin film cell technology is able to match these requirements, and (3) what tasks need to be

undertaken to make possible the goals of this thin film program.

One general area of work is to develop an improved understanding of cells and their

structures. This is understandable from the relative state of immaturity of these

developing cells and materials. The second general area of work is to develop an improved

measurements base. It should be recognized that no research and development program,

certainly not one with as many elements and participants as the thin film program, can

expect to be executed efficiently and effectively without such a supporting measurement

base. Without this base, there is no way to measure and evaluate progress with confidence

and be assured that past successes can be reproduced. It is also important to recognize

that both areas of work must be pursued together because progress in one depends and feeds

on the advances in the other.

The following two sections list important tasks in the two major areas of work which

were identified by the participants of the workshop.

2
4. An Improved Understanding of Cells and Their Structures

In their presentations and discussion groups, the workshop participants identified many

tasks to achieve the necessary understanding of cells and their structures. They stated

that such work is needed to understand better the operating principles of the new and

developing thin film cells, and of the material, processing, and assembling factors that can affect cell performance and stability.

The need to understand better the operating principles of thin film cells in general

was discussed. Polycrystalline Si, MIS, and conducting oxide on Si cell types were specifically mentioned. To this was added the need to clarify what cell and material

parameters are most relevant to cell efficiency. Certainly, the perception of these needs

is an indication of the relative immaturity of these and other developmental thin film

types. It can also be interpreted to mean that a more equitable balance needs to be struck

between efforts to understand the physics of these cells and the urgent work underway to increase cell efficiency as rapidly as possible. Achieving such a balance by giving greater

attention to the former can aid in achieving the goal of the latter.

The need to understand better the effect of material factors such as grain boundaries,

polycrystalline silicon charcteristics , and various MIS and conducting oxide material

systems on cell performance was cited. Participants also recommended that a better

understanding was needed of the various Schottky barrier GaAs systems so that those with the

greatest potential for high reliability could be identified. This would avoid wasting time

in trying to develop systems which would eventually prove to be unsuitable.

There are a number of parts of the cell structure which need to be better understood

and improved: low-cost substrates for GaAs AMOS devices, interfacial oxides, metal oxide

windows, barrier metal alloys, and encapsulations. However, by far the most often repeated

interest and concern expressed was in regard to diffusion, interdiffusion, and surface

reaction phenomena affecting virtually every interface of the various thin film cells.

Mentioned were diffusion of metals from grid contacts into GaAs and Cu-ternaries ; diffusion

of electrically active defects through Cu-ternaries; general compatibility problems of

Cu-ternaries with adjacent cell components; growth of interfacial layers due to oxygen

diffusion through and from conducting oxides and through CU2 S overlayers; effects of grain

boundaries on diffusion coefficients; effect on Cu nucleation of the structural quality of

CU2 S; and generalized interactions of window materials with adjacent materials.

Deficient knowledge of the component materials and their interactions is reflected in concerns expressed about the need to understand better how various processing and assembling

procedures can affect cell performance. How and why does cell performance depend on, for

example, the method of deposition, growth, and treatment of Cu-ternaries, and on the method

of application of oxides and metallizations in MIS cells?

Three aspects related to the assembly of thin film cells were highlighted which bear

directly on the ability to achieve the 20-year life goals of the thin film solar cell

program. They are hermeticity, encapsulation, and contacts. Ways need to be found to define and improve the quality of hermetic protection of these cells. There are questions

about encapsulations and their ability to sustain long term exposure and to maintain good

bonds to the device structure. Some of these questions may be answered by experience gained

in the low cost solar array program. Finally, there is concern about not only assuring that

good electrical contacts can be maintained for metallization grids but about how to make

good electrical contacts to thin films without degrading device performance.

5. An Improved Measurements Base

The participants cited many tasks that will need to be completed before a satisfactory

measurement standards base for thin film cells can be established.

There are a number of problems that need to be solved if accelerated tests are to be

used in a meaningful way. One is the need to develop clear functional relationships between

test stresses and degradation mechanisms. Another is the necessity to incorporate the

synergistic effects that different stresses may have on cells. To be able to do this

requires detailed knowledge about the degradation mechanisms. This can evolve out of work

3
described in the previous section. Added to these is the observation made that adequate

measurement precision in accelerated testing is very important. This is because of the

necessity to predict from small parameter changes, during the time of the accelerated test,

the period of useful life.

Specific material characteristics were identified for which test standards and test development are needed. In particular, standards for various polycrystalline silicon

characteristics such as resistivity, mobility, lifetime, and diffusion length were identified, as well as standards for measuring grain size and orientation, void space

between grains, and impurity within grains and at grain boundaries. Methods need to be

developed to measure and characterize interfacial layers in MIS cells for thickness (with

time), pin holes, defects, impurities, stoichiometry variations, and for the density and

energy of charge states.

Finally, the need to establish guidelines for reporting results was emphasized. To serve as a starting point, one discussion group developed a list of test conditions that

need to be specified (i.e., insolation, thermal, stress, electrical loading, and ambient).

6. In Conclusion

As progress is made in the cell characterization work outlined in section 4, a natural

outcome will be a better understanding of (1) the degradation mechanisms of these cells and

(2) the requirements for achieving the degree of control over cell processing and assembling

steps needed to obtain greater cell performance uniformity. Armed with this knowledge and

control and the improved measurements base that can be developed from the other area of work

outlined in section 5, it will be possible to make more meaningful estimates of the long

terra stability of these cells and to make more knowledgeable decisions about material,

device, and process design improvements to achieve the performance goals for efficiency and

long term stability of the thin-film solar cell program.

4
APPENDIX

HIGHLIGHTS OF WORKSHOP SESSIONS

The workshop was divided into three sessions. In the first, researchers reviewed modes

and mechanisms for degradation and failure and the status of present reliability testing

pertinent to the three groupings. In the second session, speakers from related device

technologies discussed measurement and test approaches that they have used for achieving

high reliability in their device areas. Three discussion groups were formed for the third

session, one for each of the three cell and material classifications. The task of these

groups was to identify what means are available and what is to be done to satisfy needs in

achieving goals for long term stability of advanced-technology solar cells. Reports of

these working groups were presented by the group chairmen at the final session of the

workshop. Highlights of the presentations and discussion groups follow. The title and

author(s) of each presentation are located in the margin adjacent to the highlight.

Session I: Status of Present Reliability Testing, Failure Modes, Failure Mechanisms, and

Data for Thin Film Solar Cells and Materials

The primary degradation mechanisms of the Cu2 S/CdS heterojunction cell are the oxidation of CU2 S, diffusion of Cu into

CdS, electrolytic decomposition of CU2 S to nucleate copper, and

loss of structural integrity of electrical contacts and device/

encapsulant interfaces due to thermal expansion mismatch and UVinduced bond strength degradation, respectively.

None of these degradation mechanisms preclude 10- to 20-year

life, but the measurement of the rate of degradation of these

cells and hence the prediction of life is a problem. To use ac- celerated test procedures to predict life requires a detailed

knowledge of the mechanisms being accelerated. For thin film

solar cells in general and for Cu2 S/CdS cells in particular,

this information is not available. Meaningful accelerated testing

will not be possible until accelerated tests are matched to specific degradation mechanisms in a knowledgeable way.

The following four major research needs for C^S/CdS solar

cells were identified: (1) Work is needed to improve and define

the quality of the cell's hermetic protection, because these cells

will degrade rapidly if exposed to air. This effort should also

include the development of a quantitative understanding of the ox- idation of the CU2 S layer and the resultant structural and electronic changes. (2) The interdiffusion mechanisms which reduce

cell performance need to be identified. (3) The relation of various structurally related degradation modes, such as encapsulation

delamination and loss of grid contact, to use-conditions needs to be identified. And, (4) the relationship between copper nodule

formation (due to electrolytic decomposition of CU2 S) and the

structural quality of the CdS layer needs to be identified in detail so that the level of CU2 S quality required for 10- to 20-

year life can be established.

Work on the copper ternaries (CuInS 2 » CuInSe 2 , and CuInTe2

) with CdS for photovoltaic devices is in its infancy. Relatively

little is known about the compatibility of these materials in a device context and much materials work needs to be done. More

work is also needed to understand how methods of deposition,

growth, and treatment of ternary films critically affect the

cell's structural and electrical characteristics and their relevancy to cell stability.

The Stability and Relia- bility of CdS/Cu2S Solar

Cells, J. D. Meakin and J.

E. Phillips, Univ. of Delaware

Stability and Ternary

Chalcopyrite Photovoltaic

Devices, L. L. Karraerski, Solar Energy Research Institute

5
Stability of Thin Film Polycrystalline Silicon

Solar Cells, T. L. Chu,

S . S . Chu, and E . 0. Stokes, Southern Methodist University

Reliability Studies on MIS and Transparent Oxide-Si Solar Cells, W. A. Anderson and J. K.

Kira, Rutgers University

Major causes for Cu-ternary/CdS device degradation are various impurity diffusion and interdiffusion reactions, which are aggravated by grain boundaries in the polycrystalline films. The

high diffusion coefficients reported for Cd and other elements in

Cu-ternary/CdS heterostructures, because of the number of vacancies in the ternary, pose potential reliability problems. Fast diffusion of metals into the ternaries can undermine contact sta- bility. Diffusion of oxygen from the ambient during annealing can cause an unwanted oxide layer to form at the interface between CdS

and the Cu-ternary. And, rapid diffusion of electrically active

defects can cause a rapid increase in electrical conductivity of

the ternary.

Preliminary results of the stability of thin film polycrystalline silicon solar cells prepared on metallurgical silicon substrates were described.

The temperature coefficients of the cell parameters are similar to those for single crystal silicon solar cells. Exposure

to 115°C for up to 160 h, temperature cycles from -40°C to 100°C,

and high humidity resulted in no significant change in cell

parameters. Exposure to an irradiance of 1.2 W/cm^ at 150°C

under load for 150 h showed some degradation due presumably to a deterioration of grid contacts.

The results of preliminary studies of the stability and degradation of MIS (silicon) and metal-oxide-on-silicon heterojunction* solar cells was described. The few previously reported

results of degradation studies of MIS cells using silicon were re- viewed, recent environmental stress and Auger analysis data on Cr/oxide/Si systems were reported, and some degradation results

obtained by R. L. Anderson of the conducting metal oxide materials

Sn02 > I^Oj, and I^C^SnC^ (indium-tin oxide) on silicon cells

were mentioned.

The degradation of tin-oxide-on-silicon heterojunction solar

cells is attributed to field assisted transport of 02 ~ from

the metal oxide to the silicon to form a SiC>2 layer at the interface and adversely affect the series resistance, fill factor,

and Voc of these cells. Aluminum contact degradation has also

been observed.

The major degradation problem in MIS cells is the alteration

of the oxide interfacial layer due to exposure to air and moisture. Examples of the growth of layer thickness beyond optimum

thickness were shown for Au on n-type Si and for A1 on p-type Si

where moisture trapped in the oxide during formation led to the

growth of an aluminum oxide.

Some general observations were made: Hermetic sealing of MIS

cells is needed to prevent air and moisture from degrading the oxide interfacial layer and also from degrading the conductance of

thin metal films and contacts to these films. Encapsulants are needed which do not discolor or soil during extensive outdoor use. Also, encapsulants need to be studied for their long term performance.

^ • • The metal oxide forms a junction with the semiconductor and also acts as a low resistance, transparent contact.

6
Extensive results of the degradation of conducting oxides on silicon cells were described. Both hetero juncti on and heteroface

(oxide serves as window only) cells were considered.

The prominant degradation mechanism for SnC>2 on Si heterojunction cells is the formation of a high resistance SiC>2 layer

at the Sn02~Si interface at room temperature. In2

C> 3 -Si interfaces also degrade in this way but only at elevated temperatures.

For both conducting metal oxides, the degradation proceeds more rapidly for polycrystalline silicon than for single crystal cells.

Degradation of the : SnC^-Si interface for both heterojunction and heteroface cells does not occur at temperatures below

470°C. This high threshold temperature allows the annealing of

process-induced defects in the silicon and makes promising the use

of this conducting oxide for both n- and p- type Si heteroface

cells.

Degradation of aluminum electrical contacts to the conducting

metal oxides can be prevented by using a chromium barrier.

While the MIS solar cell design holds promise for use with

polycrystalline and amorphous semiconductors, there are a number

of problems whose importance is affected by the material system

selected and fabrication process used. These problems are related

to chemical and electronic effects at the insulator-semiconductor

interface and film structure effects.

Stability of Conducting

Oxide/Si Heterostructure

Solar Cells, S. L. Franz,

M. L. Andren, and R. L. Anderson, Syracuse Uni- versity

Unique Problem Areas in M-I-S Solar Cell Structures, S. J. Fonash, G. Fishkorn, and T. E. Sullivan, Pennsylvania State University

The electronic nature of the semiconductor surface can undergo considerable changes as an oxide is grown or an insulator is deposited. These changes can be desirable or undesirable depending on the material system used. The problems encountered with a Pd-oxide-n-type Si system were described.

The performance of an MIS cell can be strongly affected by

the rate, method, and material of the metal film deposition as well as the surface condition of the semiconductor. A case in

point is that while Ag films of good quality can be deposited on

Si, Ag deposited on Si having 20 to 30 nm of oxide can lead to a porous film. Such a film leads to high sheet resistance and provides openings for covering materials and the ambient to reach and

degrade the junction.

The selection of the semiconductor can also be important:

while considerable slow trapping of charge induced by current and

causing hysteresis effects has been observed in GaAs , it has not for Si-based systems.

A polycrystalline GaAs/Ge/s t ee 1 AMOS solar cell system now

under development at JPL was described. Two areas under exploration are the passivation of the Ge/steel interface and various

combinations of barrier metals and interfacial oxides. Life testing with several metal-oxide combinations has only recently been

initiated using 100°C laboratory air environments. Examples were given of cases where the choice of barrier metals such as Ag, Ni,

and Cu can be important not only for cell reliability but for efficiency as well. The choice of oxides such as native oxide,

M0O3 , and Sb 2C>3 can have marked effects on the cell Voc , fill factor, and stability. Additional experiments are needed (1) to study other deposited oxides with Ga-saturated metals, particularly with reproducible stoichiometric oxides, and ( 2 ) to identify

metal-oxide-GaAs systems with potential high reliability.

Reliability Testing of GaAs AMOS Solar Cells, R. J. Stirn, Jet Propulsion

Laboratory

7
Stability of Thin Film Polycrystalline Silicon Solar Cells, T. L. Chu,

S . S . Chu, and E . D

.

Stokes, Southern Methodist University

Stability Studies of Amorphous Silicon Solar

Cells, D. E. Carlson, RCA Laboratories

Preliminary measurement results of the temperature dependence

and stability of polycrystalline GaAs solar cells were reported.

The change with temperature of V oc and conversion efficiency

of gold barrier MIS polycrystalline GaAs cells were found to be

similar to those of single crystal GaAs p-n junction cells, while

silver barrier cells were found to have higher temperature coefficients. Storage at 60°C for 24 h left the I-V characteristics of

gold-barrier cells essentially unchanged but left those of the

silver-barrier cells with evidence of increased series resistance.

Gold barrier cells were operated under an irradiance stress of

^325 mW/cm^ at a temperature between 50 and 55°C for more than

48 h. While some degradation of room temperature cell performance

was noted immediately after stress, complete recovery was noted

after a few hours.

Hydrogenated amorphous silicon solar cells should be stable

under normal operating conditions. The results of a number of ex- periments to explore the stability of amorphous silicon were described. The evolution of hydrogen (applicable at temperatures

above about 350°C) will not be a problem at 100°C. The diffusion

of dopants such as P and B will not be a problem even at deposition temperatures as high as 300 to 350°C. Electrode materials

such as Cr, Mo, Nb, and Ta exhibit little evidence of diffusion,

even at temperatures of 400°C. The use of A1 and Fe, however,

will require care. Iron silicides may form at a fabrication tem- perature of 400°C to create a large series resistance, while at 300°C the diffusion coefficient is low enough to prevent this formation. Aluminum interdi f fuses at 300°C and has been reported to induce crystallization of amorphous silicon at 335°C. The presence of an oxide layer on A1 inhibits this interdiffusion, but it also creates a large series resistance.

Good stability of devices made with Schottky barriers and of

p-i-n cells using other than native oxides was reported. MIS devices utilizing the native oxide as a thin insulating layer exhibit degradation when exposed to air, but this can be reversed by

heat treatment.

Session II: Measurements and Tests Used to Define Stability in Related Technologies

Silicon Cell Space Pro- gram Experience, P. A. lies, Optical Coating

Laboratory, Inc.

The experience of the silicon cell space program and its implications for thin film cells were presented. Most of the methods used to ensure stable space cells are ruled out for terrestrial application because of the costs involved. On the other

hand, efforts made in the space cell program to develop a spirit

of cooperation between cell makers and users and to select realistic tests will be even more important in the thin film cell program.

The wide range of possible materials and processes for thin

film cells will make difficult the early utilization of automated

production (which will probably be needed to achieve low-cost

goals). It will also make complex the task of developing tests to measure the stability of these many cell types.

The use of thin film materials can present major problems in

attempting to achieve long cell life. For all thin film cells,

the contacts will pose a major problem due to initial requirements

(low cost and good conductivity) and stability needs (freedom from

corrosion and failure under temperature cycling). Also, problems

should be expected from interlayer movements of material during

cell life, with their adverse effects on cell properties.

8
Effective methods of encapsulation will need to be developed

to reduce the degradation effects of the atmosphere on cells.

Also, the search for met allurgi cally stable systems must be

continued.

Reliability concerns and life test approaches for concentrator solar cells were described. Reviewed were the many stressful

operating and environmental conditions that concentrator cells

must sustain which hold the possibility of many causes for degradation. Just how serious these stressful conditions may be re- mains to be seen because only limited life test data are presently

avai lable

.

Reliability Concerns and Life Test Procedures for Concentrator Solar Cells,

W. V. McLevige, Sandi

a

Laboratories

The three most basic accelerated tests used and ones that were recommended for consideration are those involving constant

temperatures, temperature-humidity cycling, and temperatureillumination stresses. Some preliminary results were described of

recent accelerated tests on Si and AlGaAs/GaAs concentrator solar

cells. These examples were provided for their tutorial value in

demonstrating the kind of information that can be obtained from

such tests.

Some of the reliability problems encountered in integrated

circuits were reviewed to alert the photovoltaic community of potential pitfalls and to advise it of ways to view the reliability

problems to help achieve solar cell stability.

Proper device design and fabrication are not only necessary

to achieve the desired functional performance but also the desired

reliability. The need to assure that reliability is built in the

device has led to, for example, the use of the scanning electron

microscope to check the quality of metallization interconnections.

The introduction of such a sophisticated instrument on a manufacturing processing line was revolutionary at the time, but its use

is now an accepted fact. The ability to detect problems with this

instrument early in the manufacturing process led to production

cost savings and to process changes that have resulted in inherently more reliable devices.

In summary, the importance of user involvement was emphasized. Involvement means a heightened awareness of device design,

processing, manufacturing, testing, and device specification. Additionally, it means the development of capabilities to perform

product evaluations and failure analyses. And, it implies coordination and cooperation not only with other users but with manufacturers themselves to promote product reliability and standardization.

Some Reliability Problems

in Integrated Circuits - Their Detection, Definition, and Remedy, J. W. Adolphson, NASA/Goddard

Space Flight Center

The major degradation and drift phenomena of integrated circuits and the tests which can be used to uncover them were re- viewed. While these tests can reveal problems, sophisticated analytical techniques are necessary to thoroughly understand and to control the processing and material parameters which lead to instabilities and degradation.

Most of the instabilities encountered in integrated circuits

are functions of electrical bias, humidity, or temperature and can

be accelerated by subjecting the devices to the application of ex- traordinary levels of these stresses. Tables were reviewed which

summarize the types of tests expected to be useful in identifying

devices afflicted by the various drift and degradation mechanisms .

Tests for Instabilities

in Silicon Integrated

Circuits, C. W. Green,

Bell Telephone Laboratories

9
Real-Time Controls for Reliability Assurance, S. Kukunaris, RCA

Interdiffusion and Interface Problems Relating to Thin Film Photovoltaic

Devices, L. L. Kazmerski,

Solar Energy Research In- stitute

Corrosion and Its Control, R. P. Frankenthal,

Bell Laboratories

Because of the complexity of integrated circuits, it is impractical to subject all sensitive components to stressful conditions of accelerated life tests to predict long field service.

Insight into device quality and reliability is acquired from a combination of using selected stress tests, thorough analysis of

the component materials and processes, and assurances that the

quality of materials and the processes and devices remain con- trolled.

An approach to device stability analysis which can be accom- plished quickly and with few samples was described; this approach

has been used with transistors and CMOS devices. It is called

real-time control testing, and its basis is the use of accelerated

testing where the level of stress is selected to maximize the ac- celeration of degradation mechanisms active during device life.

The stress level is selected to be as high as possible without ac- tivating failure mechanisms not active during the operational life

of the devices tested. This stress level can be selected with as- surance only when an information bank is available which consists

of the results of extensive traditional efforts in life testing, stress testing, failure analysis, and field use.

Interdiffusion and interface problems were reviewed which are related to thin film polycrystalline photovoltaic device performance and stability. These problems are aggravated by enhanced

interactions at grain boundaries. It was cautioned that while the

diffusion processes have been much studied by a variety of techniques, these studies have sometimes been conducted with mixed

success and skill. The relatively recent development of surface

analysis techniques and the use of two in particular, Auger electron spectroscopy and secondary ion mass spectroscopy, have highlighted interdiffusion investigations, however.

The three major interface problem areas were considered: the

grid contact to the semiconductor, the semiconductor to semiconductor junction, and the semiconductor to back contact region.

Representative problems were cited for a number of material systems: InP/CdS, GaAs, amorphous Si, CU2O, C^S/CdS, Cuternaries, and SnOx /Si. With these examples, it was concluded

that much work needs to be done in controlling interdiffusion

mechanisms in thin film polycrystalline solar cells so that these

degradation processes can be minimized.

The conditions for which electrolytic, galvanic, and stress corrosion can develop were reviewed. Many corrosion problems can

be avoided by the proper choice of materials and device design,

and by rigorous efforts to avoid moisture and ionic contamination.

Moisture can reach the device surfaces either as a result of ex- posure of parts to moisture prior to or during device processing,

by diffusion through encapsulants (most encapsulants used are permeable to water), or by ingress through faulty seals. Only minute

amounts of ionic contaminant residues on surfaces, due to inadequate cleaning procedures, can greatly increase the rate of corrosion where moisture exists.

Corrosion failure modes mentioned are increased resistance of

conductor stripes, short circuits due to dendritic formations, and

increased leakage currents due to the spreading of corrosion products between conductors.

Two other noteworthy observations were made. No metal is im- mune from corrosion, given the right conditions; and many in10
stances of corrosion can be attributed to dust particles from the

atmosphere.

Experiences in the Low-Cost Solar Array Project with silicon

flat-plate solar cell modules were described. Six key failure

modes have been identified from field experience. They are: electrical interconnect breakage, metallization deterioration,

electrical insulation breakdown, solar cell cracking, encapsulation cracking and delamination, and optical surface soiling. It is probable that they will be important in thin film arrays also.

Problems with the first three have been substantially solved,

while considerable progress has been made with the others.

Qualification tests have been used to evaluate design solutions to address various failure modes. Some of the problems in

the selection of meaningful environmental test stresses were discussed, and the relative value of the environmental tests used were reviewed. Thermal cycle, humidity, and structural tests have

proved to be of great value, while ultraviolet stress tests have

been difficult to interpret. Research on ultraviolet-humidity,

bias-humidity, and module soiling tests is under way.

Recently proposed concepts and methodology for designing ac- celerated tests for silicon solar cells were highlighted. The

basic approach is equally valid for the accelerated testing of

thin film cells, although accelerated tests for thin film cells

will likely have to be designed to be specific to each of the different cell types. Among the aspects of this design approach is the requirement of using only "mature" devices made by wellcontrolled fabrication processes, the availability of a broad

range of data on device experience, engineering judgments based on these data to assess the relative severity of various combinations

of stresses that need to be considered, an approach for uncoupling

different failure mechanisms, a suggested time-acceleration factor

of ten, and the use of at least five stress levels for each type

of stress.

Terrestrial Silicon Array

Field and Test Experience, R. G. Ross, Jr.,

Jet Propulsion Laboratory

Methodology for Designing Accelerated Aging Tests

for Predicting Life of Photovoltaic Arrays, R. E. Thomas and G. 8. Gaines, Battelle-Columbus Laboratories

Session III: Discussion Group Sessions to Identify Tests and Measurement Procedures That

Can be Used to Enhance the Prediction of Material and Device Stability

Several general aspects of measuring and reporting cell and

material characteristics and stability were discussed. As a guide

to what to expect of the types of cells under discussion, the following five classes of degradation mechanisms were developed and

discussed: interdiffusion (implies mass transfer), chemical (oxidation, corrosion, or other chemical state change), electrolytic

(decomposition due to field-induced ion motion), mechanical (such

as grid lift-off, delamination, etc.), and photochemical (which

encompasses any cell change which is directly induced by photons )

.

Discussion Group I on Cu2 S/(CdZn]S and Amorphous Si, L. L. Kazraer- ski, Solar Energy Re- search Institute, and J.

D. Meakin, Univ. of Delaware

To allow for the development of a meaningful data base on which to assess test results of others and to evaluate cell stability, substantial agreement was reached in defining what conditions should be reported and the circumstances under which it is necessary to specify the test method details. Some of the general

test conditions identified are insolation, temperature, stress,

electrical loading, and ambient. Also cited was the importance of

providing complete information about the experimental design and

the characteristics of the cells before the initiation of the

11
tests. The group also felt that to intercompare the performance

of cells from different organizations and production periods, it would be better to establish a set of test procedures to which all

would adhere than to establish a central test facility, as has

been suggested by some.

A number of aspects and considerations regarding accelerated

tests were discussed. To be able to project long term performance

of cells, it will be necessary to establish functional relationships between test stresses and the degradation mechanisms to be

accentuated. Also, the synergistic effects of different stresses

to a given degradation mechanism cannot be ignored. Furthermore,

the need to extrapolate long term cell stability from small

changes in characteristics resulting from stress tests over a relatively short testing time places considerable importance on the

precision of the test methods used to measure these changes. The

paper presented in Session II on the methodology for designing ac- celerated aging tests for degradation mechanisms stimulated con- siderable discussion about its application to thin film cells.

While the potential value of this approach was appreciated, some members of the group felt that the use of five stress levels,

which was recommended, would require an excessive amount of testing.

Discussion Group II on Polycrystalline Si, MIS, and Conducting Oxide/Si,

J. Schewchum, McMaster University, and R. L. Anderson, Syracuse Uni- versity

An extensive and far-reaching set of research needs was developed which reflects both the breadth of cell designs and the

inmaturity of the material, cell, and measurement technologies involved. What is needed, basically, is a better understanding of

and an improved measurement capability for virtually every aspect

of these cells.

The material parameters of the various classes of polycrystalline silicon pertinent to solar cell performance need to be

identified. Measurement standards of polycrystalline silicon are needed to allow for more meaningful assessment of data being re- ported. In particular, methods to measure the following were

cited: resistivity, mobility, lifetime, diffusion length, grain

size and orientation, void space between grains, and impurity content within grains and at grain boundaries.

Guidelines are needed to measure the various aspects of interfacial layers such as interfacial resistance, variation of

thickness with time, pinholes, impurity composition, and density

and energy of charge states.

A better understanding is needed of the various reactions at interfaces; of the influence of grain boundaries on the mechanical

and electrical properties of materials and cells; of the effects

of built-in and applied electric fields; of the compatibility of

the various material combinations; of interdiffusion phenomena at interfacial layers, window interfaces, and grain boundaries; of

conducting window materials; and of the encapsulants used.

Indicative of the relative immaturity of the technology for

these cells, the need to understand better the operation of these

cells and to understand their degradation mechanisms was emphasized. As a consequence, there is a need to identify which parameters affect most the energy conversion efficiency of these cells.

Also, ways are needed to predict the potential conversion efficiency of various material systems to determine which are worth developing.

12
Regarding the matter of long term stability, it was felt that

all mechanisms encountered in single crystal homojunction solar

cells must be considered. To these must be added degradation

mechanisms related to any possible incompatibilities of the

various materials used, including encapsulant materials. The

classes of degradation mechanisms identified by the group are essentially identical to those identified by Discussion Group I. Because of the great differences in the material systems, it will

not be possible to have accelerated test standards applicable to all cell types. Initial approaches in developing these standards

should, however, be based on those for single crystal silicon

cells. To accelerate the development of a data base, laboratory

and use tests should be conducted concurrently. Tests on unprotected cells, insofar as that is possible, should be made to assist in discovering intrinsic degradation mechanisms.

Thin film GaAs solar cell technology was considered to be

sufficiently immature to require the discussions to be oriented

more to understanding the various mechanisms at play in these

cells than to address the question of predicting cell lifetime.

Three broad topic areas were examined: classes of degradation,

test strategies to uncover degradation problems, and areas where

more research is recommended.

Regarding degradation, three major classes of phenomena were identified and discussed: interdiffusion, chemical, and mechanical.

Discussion Group III on Thin Film GaAs, R. J. Stirn, Jet Propulsion

Laboratory

Under interdiffusion phenomena are a plethora of effects, for

example: degradation of the contacts between the GaAs and the

substrate and the front metallization; reactions involving the

barrier metal, interfacial oxide, and bulk GaAs; degradation of

conducting oxide layers; and reactions at grain boundaries. Under

the category of chemical effects were mentioned front contact corrosion and inter facial oxide modification problems. Under mechanical effects were potential problems with the strength and integrity of the front contacts and interconnects, especially in

Schottky barrier-type cells where bonds to a thin metal film are required

.

A number of screen, operating, and accelerated tests were suggested to reveal some of the above degradation modes. They involved the use of I-V characteristics, as a function of temperature, and the use of changes in spectral response to reveal cell

degradation, in the process of subjecting cells to temperature

cycling and high temperature storage stresses. High humidity is

added to accentuate those stresses.

Four general areas of research were called out as requiring

extra attention to promote the development of viable cells. The

most crucial area is in the selection of the low cost substrate

and its treatment. Research is also needed to better understand

the interfacial oxide layer and barrier metal alloys of these

cells, to investigate more cost-effective means of manufacturing

GaAs, and to study the making of electrical contacts to thin films

with adequate bond strength and low contact resistance that do not lead to shorted junctions or degraded Schottky barriers.

13
NBS-U4A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1778

4. TITLE AND SUBTITLE

A Perspective of a Workshop on Stability of (Thin Film)

Solar Cells and Materials

5. Publication Date June 1979

7. AUTHOR(S)

Harry A. Schafft and David E. Sawyer

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. Task Order A05^“SE

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Advanced Materials R&D Branch

Division of Distributed Solar Technology

Department of Energy

20 Massachusetts Avenue, N.W., Washington, DC 205^5

13. Type of Report 8< Period Covered

Final

15. SUPPLEMENTARY NOTES

I | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas factual summary of most significant Information. If document Includes a significant bibliography or

literature survey, mention it here,) The results of a workshop on the Stability of (Thin Film) Solar Cells and

Materials are reviewed from a measurements perspective. Solar cells of

the following three material groupings were considered at the workshop:

(1) Cu 2 S/[CdZn]S, Cu-ternar ies/CdS, InP/CdS, and amorphous Si; (2) polycrystalline, MIS, and conducting-oxide Si; and, (3) polycrystalline and

AMOS GaAs. Considering the relative state of immaturity of these developing cells and the goal for high reliability and stability, two general areas of work are recommended. One is to develop an improved understanding of cell operation and of component structures of these cells.

The other is to develop an improved measurements base'. Specific needs

and recommendations are provided.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the firet letter of the firet key word unleee a proper name;

separated by semicolons) Accelerated tests; failure mechanisms; failure modes; photovol tai cs ; reliability;

semiconductors; solar cells; stability; testing; thin films.

18. AVAILABILITY [X] Unlimited

I I For Official Distribution. Do Hot Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- Rl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

15

22. Price

$4.00

USCOMM-DC